Depends on your skin type how long you will take to get a even tan, Mine took about a month to lose the white patch, and if you have never shaved your head before the skin has to get used to it`s new found freedom, and will take longer to tan, but keep shaved and let air at your head and it will tan, But get out in the sun as much as possible,
In the winter months i don`t use any sun screen, but in summer use spf 30, but i have shaved my head for 7 yrs and the skin is toughened up and sun dosn`t bother it much now.
Then as for sun screen the higher the spf rating the better, when swimming etc make sure you regularly apply sun screen as this is when you burn the worst